1. Field of the Invention
This invention relates to hydrogen peroxide-free and hydrofluoric acid-free coating compositions used in the autodeposition of resinous coatings on iron, zinc, and their alloys.
2. Statement of the Related Art
Compositions and processes for the autodeposition coating of organic polymers on metals are now well known, having been disclosed in related U.S. Pat. Nos. 3,585,084 and 3,592,699 to Steinbrecher and Hall, both assigned to Amchem Products, Inc., Ambler, Penn., U.S.A., as well as in later U.S. Pat. No. 3,709,743, among others.
These patents, as well as U.S. Pat. No. 4,373,050, to Steinbrecher and Hall, also assigned to Amchem Products, Inc., all disclose the need for an acid, preferably hydrofluoric, and an activating compound, which is generally an oxygen-containing oxidizing agent. For the purpose of this invention, "oxidizing agent" or "oxidizer" is defined as a substance which has, or promotes in aqueous solution, a positive reduction potential greater than that of hydrogen, and which need not itself contain oxygen.
U.S. Pat. No. 3,585,084 discloses using hydrofluoric acid and, as utilizable oxidizing agents, hydrogen peroxide, dichromate, perborate, bromate, chlorate, permanganate, nitrite, and nitrate, in the form of alkali and ammonium salts; hydrogen peroxide and dichromates being preferred and used as the oxidizing agent in every example except Example 87, which discloses using nitric acid as both an oxidizing agent and a source of acid for the bath composition.
U.S. Pat. No. 3,592,699 discloses using hydrofluoric acid and only the use of hydrogen peroxide or dichromate ions as the oxidizing agent.
U.S. Pat. No. 3,709,743 discloses using hydrofluoric acid, and an oxidizing acid as the oxidizing agent, but only nitric acid is specifically disclosed, and is used in all the examples.
U.S. Pat. No. 4,373,050 discloses the desirability of using an oxidizing agent even in addition to an acid such as nitric acid, (which itself can function as an oxidizing agent), because of gassing problems. The same oxidizing agents as in U.S. Pat. No. 3,585,084 are disclosed, with hydrogen peroxide and dichromate salts again being preferred, the examples being essentially identical to those of U.S. Pat. No. 3,585,084.
U.S. Pat. No. 4,414,350 discloses autodeposition compositions containing hydrofluoric acid with a ferric salt such as ferric fluoride, which generates increasing amounts of ferrous ions. The concentration of ferrous ions is then controlled by adding at least one carboxylic acid, which forms a ferrous ion complex. Useful carboxylic acids include acetic, itaconic, succinic, acrylic and citric, with citric acid being preferred.
South African patent application No. 72/1146 discloses autodeposition compositions using hydrofluoric or acetic acids, and a persulfate oxidizing agent, among others mentioned above.
U.K. Pat. No. 1,241,991 discloses autodeposition compositions using p-benzoquinone and p-nitrophenol, among others mentioned above.
In the known autodeposition compositions and processes partially exemplified by the foregoing related art, gassing will occasionally occur when the oxidizing agent, particularly hydrogen peroxide which is most commonly used, reacts with iron compounds that are present. This gassing is probably the cause of discontinuities in the autodeposited coating, which in turn lead to decreased corrosion protection. Furthermore, certain latices, specifically those with polymers having C.dbd.C bonds, appear to be somewhat permeable to corrosion, possibly because of these double bonds.
Hydrogen peroxide is the most frequently mentioned oxidizer in prior art autodeposition compositions and is widely used in industry. However, hydrogen peroxide has a number of serious disadvantages, of which the following is one example.
U.S. Pat. No. 4,186,219 discloses the use of hydrogen peroxide in autodeposition compositions comprising latex, hydrofluoric acid and ferric fluoride (FeF.sub.3), in aqueous solution. In such a system, the ferric fluoride acts as the oxidizer for the iron-containing substrate metal. As the substrate is processed, the ferric fluoride is reduced to ferrous fluoride (FeF.sub.2), and substrate iron is oxidized to ferrous fluoride which results in a decrease in oxidizing potential and, if allowed to increase in concentration, will also result in destabilization of the entire coating composition. Hydrogen peroxide is added, not to oxidize the substrate, but rather to oxidize the ferrous state back to a ferric state, thus restoring the original oxidizing potential of the bath.
However, if the hydrogen peroxide is added in excess of the amount required to restore the oxidizing potential of the autodeposition bath, it will cause undue and inefficient dissolution of the iron-containing substrate, and an increase in the total amount of ferric fluoride in the coating composition. As the concentration of ferric fluoride further increases, so does the dissolution rate of the iron-containing substrate, and the autodeposition process becomes difficult to control.
Moreover, hydrogen peroxide is a very powerful oxidizer which, when added to the autodeposition coating composition to restore its original redox potential, can attack the stabilizer contained within the latex polymer dispersion and result in still further problems, such as soft coatings which show decreased resistance to water rinsing after removal of the substrate from its immersion in the autodeposition coating bath.